The present invention relates to a hydraulic reaction force apparatus for obtaining an appropriate steering force corresponding to various vehicle travel conditions such as a vehicle speed and a steering angle in a power steering system.
A power steering system for reducing a steering wheel operation force (steering force) in a vehicle must be appropriately controlled in accordance with a steering force and a steering angle which are determined by a steering operation of a driver, and vehicle travel conditions such as a vehicle speed. During a stationary swing in a vehicle stop state, or during steering at a low vehicle speed, a large auxiliary steering force is output to reduce the load acting on the steering operation. However, during high-speed travel, when a large auxiliary steering force is generated, the steering force is excessively reduced to cause the driver to feel anxiety. This is undesirable to optimize the driving feeling. In order to solve this problem, an auxiliary steering force must be reduced, in turn, to increase the steering force of the driver. Then, an appropriate force is required to steer a steering wheel to assure stability of straight travel. This steering force control must be applied in the same manner as described above when the steering angle is increased.
In order to satisfy the above needs, various types of conventional hydraulic reaction force apparatuses are available wherein appropriate rigidity (i.e., a steering reaction force) is applied to the steering wheel by a reaction force oil pressure controlled in accordance with the various travel conditions of the vehicle, thereby controlling the steering forces during high- and low-speed travel of the vehicle. A typical hydraulic reaction force apparatus is described in Japanese patent Laid-Open No. 49-102092. In a rotary control valve for flow path selection of this apparatus, a reaction force arm extends on the input shaft side. A pair of reaction force plungers are arranged on the output shaft side so as to reciprocate from both sides of the rotational direction of the arm. A hydraulic reaction chamber is formed on the outer end side of the plungers. A reaction force oil pressure is supplied to the hydraulic reaction chamber in accordance with the vehicle speed and the like to drive the plungers. A predetermined restriction force acts on the input shaft through the arm, thereby obtaining an appropriate steering reaction force and hence realizing steering corresponding to the vehicle travel conditions.
In the conventional structure described above, however, the reaction force plungers must be disposed at portions away from the axis of the output shaft but inside the output shaft. Therefore, the radial size of the reaction force apparatus is inevitably increased. This problem typically occurs when the pressure-receiving area of the plunger is increased to increase the hydraulic reaction force capacity. A power steering system with a reaction force apparatus of this type is mounted in a small space such as a space under an engine room of the vehicle. Therefore, the hydraulic reaction force apparatus must be made compact.
A conventional compact hydraulic reaction force apparatus is described in U.S. Pat. No. 4,034,825, July 12, 1977. An engaging element such as a reaction force plunger and a ball is held on the output shaft side to be movable toward the center of the shaft, and a hydraulic reaction force is applied from the outside of the engaging member. The engaging member is engaged in an engaging recess formed on the input shaft side, thereby generating a restriction force.
In such a conventional structure, if the engaging member is a plunger, the distal end of the plunger is in slidable contact with the engaging recess. When a hydraulic reaction force is generated, a sliding resistance therebetween is increased. A frictional force is increased, and operational stability of the apparatus cannot be assured. When the pressure-receiving surface of the plunger is increased, the radial size of the reaction force apparatus is undesirably increased. The apparatus cannot be made compact, and the plunger diameter is undesirably increased. The guide length becomes insufficient, and it is difficult to obtain a stable operating condition.
When the engaging member is a ball, it is difficult to obtain perfect oil-tightness between the ball and the guide hole therefore. A hydraulic reaction force leaks considerably, and the pressure-receiving surface of the ball is small. In order to obtain an effective hydraulic reaction force, a hydraulic source having a large flow rate is required, thus resulting in impractical applications. Strong demand therefore has arisen for developing a hydraulic reaction force apparatus which is free from the conventional problems, i.e., which is compact and has operation stability.